1. Field of the Invention
The present invention relates to an ink-jet printer for ejecting ink droplets through a droplet outlet orifice (a nozzle) and recording on paper and a method of driving a recording head for an ink-jet printer.
2. Description of the Related Art
Ink-jet printers for ejecting ink droplets through a droplet outlet orifice communicating with an ink chamber and recording on paper have been widely used. For such printers methods have been developed for stably reducing a droplet size so as to achieve higher resolution and for varying a droplet size dot by dot so as to produce a gray-scale image and so on.
One of the methods for reducing a droplet size is expanding an ink chamber for retracting a position of extremity of ink called meniscus inside a droplet outlet orifice towards the chamber and contracting the chamber before the meniscus return to the previous position so as to eject an ink droplet through the orifice.
For example, in Japanese Patent Application Laid-open No. 55-17589 (1980), a method is disclosed for ejecting an ink droplet through the step of increasing an ink chamber volume from an initial state and restoring the initial state. It is disclosed therein that a droplet diameter is varied with a change in displacement (an amount of increase in the ink chamber volume) in an intake step.
In another example as disclosed in Japanese Patent Application Laid-open No. 2-6137 (1990), a droplet size is controlled by changing a voltage applied for reducing the pressure inside an ink chamber and for restoring an initial state.
In Japanese Patent Application Laid-open No. 59-143652 (1984), a method is disclosed for controlling a droplet size by applying an auxiliary pulse before a primary pulse for droplet ejection for changing a meniscus position in a droplet outlet orifice.
In Japanese Patent Application Laid-open No. 5-16359 (1993), a method is disclosed for controlling a droplet size by applying an auxiliary pulse and then a primary pulse in synchronous with a residual pressure wave in an ink chamber.
In such ink-jet printers, a recording head ejects droplets while traveling in the direction orthogonal to the direction in which paper is carried. Therefore, if velocities of ejected droplets vary, positions in which the droplets land vary as well. The quality of image recorded is thereby significantly degraded. It is thus important to maintain a velocity of ejected droplets constant for achieving a high quality recorded image.
A recording head is usually controlled through the use of a head carriage drive motor and the like so as to reciprocate at a constant speed. However, speed variations due to mechanical factors and a shift in distance between the recording head and a landing point of droplet may occur. In these cases errors are produced in landing points of droplets ejected from the recording head. These errors may reduce the quality of image reproduction. It is therefore desirable that a velocity of ejected droplet is controlled so as to compensate the factors for such errors.
In such a recording head in general, as described above, the ink chamber is expanded so as to retract the meniscus position inside the nozzle towards the chamber and then contracting the chamber to eject a droplet. In this case oscillations called Helmholtz natural oscillations are produced in the chamber by driving piezoelectric diaphragm. The meniscus position retracted towards the chamber is oscillated as well at the frequency of the natural oscillations. Accordingly, timing of ink chamber contraction greatly affects not only a droplet size but also a velocity of ejected droplet. Methods of driving a recording head less susceptible to such natural oscillations have been therefore developed.
In U.S. Pat. No. 4,646,106, for example, a drive method is disclosed wherein an ink chamber is contracted for ejecting a droplet at the instant when the meniscus position is retracted to the deepest position.
Another example disclosed in Japanese Patent Application Laid-open No. 8-267739 (1996) is an ink-jet recording apparatus for ejecting a droplet within time which is two thirds of natural oscillation frequency of the meniscus.
However, in Japanese Patent Application Laid-open No. 55-17589 (1980) mentioned above, it is only disclosed that a droplet size is changeable by varying an amount of displacement in the intake step while no specific drive method is described for controlling a droplet size. It is therefore difficult to precisely control a droplet size.
The method disclosed in Japanese Patent Application Laid-open No. 2-6137 (1990) is controlling a droplet size by changing a voltage applied for reducing the pressure inside an ink chamber and for restoring an initial state. However, no explanation is given to control of meniscus retraction position considering ink feed. Precise control of droplet size is practically difficult.
The methods disclosed in Patent Application Laid-open Nos. 59-143652 (1984) and 5-16359 (1995) are both applying a primary pulse after controlling the meniscus position in the nozzle with an auxiliary pulse. Therefore both methods require an auxiliary pulse. In the methods the meniscus position changes depending on the width and height of the auxiliary pulse and the time interval between the auxiliary pulse and the primary pulse. It is therefore required to adjust the plurality of parameters. Furthermore, in the former publication, the relationship between the auxiliary pulse and droplet size is not clearly described. In the latter publication, although the relationship between the droplet size and the variation cycle of meniscus position is described, no specific explanation is given to the relationship between the droplet size and the meniscus position retracted into the nozzle. Precise control of droplet size through these methods is therefore practically difficult.
As thus described, precise control of droplet size is difficult with ink-jet printers of related art. It is therefore difficult to achieve higher resolution and high quality image representation of halftone.
In the methods disclosed in U.S. Pat. No. 4,646,106 and Japanese Patent Application Laid-open No. 8-267739 (1996), although the natural oscillations of meniscus are considered, the velocity of meniscus changing the position and the phase of meniscus are not taken into account. It is therefore difficult to precisely control the velocity of ejected droplet at a constant value. Furthermore, since the methods are provided for ejection within a limited range of natural oscillations of meniscus, the velocity obtained is thereby limited. It is therefore difficult to control the velocity as desired.
It is also difficult to control both droplet size and velocity such as controlling image density and gradation while compensating a shift in droplet landing position due to unstable velocity of the recording head as described above.